The present invention relates to an image formation method using electrophotography. More particularly, the present invention relates to an image formation method using electrophotography, capable of ensuring excellent printing properties and cleanability over a long period of time in a printing apparatus such as a printer.
An image formation method using electrophotography is in wide use in the printing apparatus such as a printer and a facsimile, and consists basically of the following four steps (1) to (4):
(1) a step of uniformly charging a photosensitive body in darkness;
(2) a step of exposing the charged photosensitive body to light to form a desired electrostatic latent image thereon;
(3) a step of forming a toner image by a developer (toner) corresponding to the electrostatic latent image; and
(4) a step of transferring the toner image on the photosensitive body to transfer paper and fixing it thereat to form an image.
Therefore, in order to carry out such an image formation method using electrophotography, as shown in FIG. 1, members such as a photosensitive drum 11, a charging device 13, an image signal exposing device 15, a developing device 17, a transfer roll 19, a cleaning blade 21 and a full area exposing device 23 are provided in the interior of a printing apparatus such as a printer.
Thus, to form an image in the printing apparatus, the charging device 13 is used to uniformly charge the rotating photosensitive drum 11 in darkness, after which the image signal exposing device 15 is used to expose the charged photosensitive drum 13 to light to thereby form a desired electrostatic latent image. Then, in such a manner as to correspond to the thus formed electrostatic latent image, the developer is supplied from the developing device 17 to form a toner image, which toner image in turn is transferred to the transfer paper and fixed thereat to form an image while removing untransferred developer by use of the cleaning blade 21.
Here, known as an example of the conventional photosensitive drum is a photosensitive drum using amorphous silicon as a photosensitive layer. Use of a transfer paper longitudinal feeding apparatus having such a photosensitive drum ensures formation of images with a stabilized printing density even in the case where by way of example 10,000 sheets or more have been printed at a printing speed of 6 sheets (A4 size transfer paper)/min.
However, the photosensitive drum using amorphous silicon as the photosensitive layer suffered from a drawback that it was generally difficult to manufacture it in a stabilized manner, with its high manufacturing costs.
An organic photoconductor (hereinafter xe2x80x9cOPCxe2x80x9d) photosensitive drum has thus been proposed which consists of an electrically conductive base and an OPC photosensitive layer formed on top of the surface thereof.
Then, the OPC photosensitive drum is characterized by the provision of a photosensitive layer made of a photosensitive material (OPC material) dispersed or carried in a binder resin, the photosensitive layer typically having the initial thickness within a range of 15 to 20 mm. Such an OPC photosensitive drum as shown in FIGS. 2 and 3 includes a single or a plurality of photosensitive layer(s) 27 disposed on top of the electrically conductive base 25. In case the photosensitive layer 27 comprises a plurality of layers, it consists typically of a charge generation layer 31 formed on the electrically conductive base 25 and of a charge transfer layer 33 further formed thereon. Then, such an OPC photosensitive drum has an advantage that it is generally easy to manufacture it, with the result that its manufacturing costs are low.
However, the conventional OPC photosensitive drum posed a problem that the photosensitive layer was easy to wear, making it substantially difficult to form images with stabilized printing density for a long period of time, for example, over 100,000 sheets at a printing speed of 6 sheets/ min using the A4 size transfer paper longitudinal feeding apparatus.
Another problem was found that in the case of using the OPC photosensitive drum the cleanability of the OPC photosensitive drum was liable to lower in a short period of time. It was therefore disadvantageously difficult to remove untransferred developer in the form of toner filming phenomena by use of the cleaning blade.
For this reason, when the OPC photosensitive drum was used as the photosensitive drum, the replacement of the OPC photosensitive drum in a short period of time was hitherto inevitable. Accordingly, such a frequent replacement of the OPC photosensitive drum led to a rise in the print costs and maintenance costs, and further went against a trend toward savings of resources with the aim of global environmental protection.
In recent years, a so-called process cartridge is also employed which has been formed by integrating the photosensitive drum, the charging device, the developing device, the cleaning blade, etc., into a single unit. In this case, replacement of the OPC photosensitive drum will need the replacement and discard of other constituent elements as well, resulting further increased economical disadvantages.
Thus, as a result of wholehearted consideration, the inventors of the present invention have found out that by forming on the electrically conductive base an OPC photosensitive layer having a relatively large thickness and by limiting to a value within a certain range the amount X of reduction in the thickness of the photosensitive layer in a relatively short period of time (by way of example, a period of time corresponding to when 10,000 sheets have been printed at a printing speed of 6 sheets/ min using the A4 size transfer paper longitudinal feeding apparatus, which may hereinafter be referred to as 10,000 sheets corresponding time) in the OPC photosensitive drum, it is possible to maintain the charging retention function in the photosensitive drum over a longer period of time (by way of example, a period of time corresponding to when 100,000 sheets have been printed at a printing speed of 6 sheets/ min using the A4 size transfer paper longitudinal feeding apparatus, which may hereinafter be referred to as 100,000 sheets corresponding time), with the result that the surface potential necessary for the image formation can be secured over a longer period of time.
It is therefore the object of the present invention to provide an image formation method using electrophotography, capable of ensuring excellent printing properties and cleanability over a long period of time even in case the OPC photosensitive drums having the photosensitive layers with variously different diameters have been used.
According to an aspect of the present invention, there is provided an image formation method using electrophotography, comprising the steps of supplying a developer by a developing device onto a photosensitive drum in rotation to form a toner image thereon, transferring the formed toner image onto a transfer paper to form an image on the transfer paper, and removing untransferred developer by use of a cleaning blade, wherein
an OPC photosensitive drum is used for the photosensitive drum and includes a photosensitive layer being formed on an electrically conductive base and having an initial thickness which is a value within a range of 20 to 50 xcexcm and wherein
the amount X of reduction in the thickness of the photosensitive layer in the OPC photosensitive drum satisfies the following expression (1)
0.5 xcexcm less than 30X/R less than 1.5 xcexcmxe2x80x83xe2x80x83(1)
X: by way of example, the amount (xcexcm) of reduction in the thickness of photosensitive layer when 10,000 sheets have been printed at printing speed of 6 sheets/min using A4 size transfer paper longitudinal feeding apparatus; and
R: diameter (mm) of OPC photosensitive drum.
In this manner, by forming on the electrically conductive base an OPC photosensitive layer having a relatively large thickness and by limiting to a value within a certain range the amount X of reduction in the thickness of the photosensitive layer in a relatively short period of time (10,000 sheets corresponding time) in the OPC photosensitive drum, it is possible to obtain excellent printing properties and cleanability even after the elapse of a long period of time (100,000 sheets corresponding time).
Since the amount of reduction in the thickness can be controlled at 10,000 sheets corresponding time, correction to within an appropriate range is easy to perform even though the value suffered from a deviation. It is to be appreciated that by changing the thickness or material of the cleaning blade or by altering the position of the cleaning blade, it is possible to adjust or control the amount of reduction in the thickness.
In the present invention, the time to measure the amount X of reduction in the thickness of the photosensitive layer is not necessarily limited to the time when 10,000 sheets of A4 size transfer paper have been printed, and the measurement could be performed at any time corresponding to the print of that transfer paper. In this respect, the same applies to the following description. Therefore, more specifically, by allowing for a balance between the measurement errors of the amount X of reduction in the thickness of the photosensitive layer and the easiness of the correction or modification of the amount X of reduction in the thickness of the photosensitive layer, it is sufficient to perform the measurement during the time when 5,000 to 15,000 sheets of A4 size transfer paper have been printed, more preferably during the time when 8,000 to 12,000 sheets have been printed, most preferably during the time when 9,000 to 11,000 sheets have been printed.
For carrying out the image formation method of the present invention, it is preferred that the amount X of reduction in the thickness is measured in a continuous or intermittent manner.
By measuring the amount X of reduction in the thickness in this manner, it is possible to more easily and precisely the value of the amount X of reduction in the thickness at 10,000 sheets corresponding time. Thus, excellent printing properties and cleanability can securely be obtained even at 100,000 sheets. corresponding time.
For carrying out the image formation method of the present invention, it is preferred that the amount X of reduction in the thickness used is a mean value of the amounts of reduction in the thickness measured at three or more points on the OPC photosensitive drum in the width direction thereof.
By measuring the amount X of reduction in the thickness in this manner, it is possible to control more easily and precisely the amount X of reduction in the thickness at 10,000 corresponding time, while allowing for the dispersion of the measurement values.
For carrying out the image formation method of the present invention, it is preferred that a rate of change expressed as (Zxe2x88x92Y)/10,000 is a value within a range of 0.00005 to 0.00015 xcexcm/sheet where, by way of example, Y is the amount (xcexcm) of reduction in the thickness of the photosensitive layer when 10,000 sheets have been printed at a printing speed of 6 sheets/min using an A4 size transfer paper longitudinal feeding apparatus, and Z is the amount (xcexcm) of reduction in the thickness of the photosensitive layer when 20,000 sheets have. been printed at a printing speed of 6 sheets/min using the A4 size transfer paper longitudinal feeding apparatus.
By controlling the change in the amount of reduction in the thickness by the rate of change in this manner, it is possible to obtain more securely excellent printing properties and cleanability at 100,000 sheets corresponding time.
For carrying out the image formation method of present invention, it is preferred that the thickness from the apex of highest protuberance lying on the surface of the electrically conductive base to the surface of the photosensitive layer is a value equal to or more than 10 xcexcm.
Due to the existence of unevenness on the surface of the electrically conductive base, the charging retention function is liable to be controlled at a region where the photosensitive layer has the minimum thickness. Thus, by controlling the thickness of a region where the photosensitive layer has the minimum thickness, in other words, the thickness from the apex of the highest protuberance lying on the surface of the electrically conductive base to the surface of the photosensitive layer, it is possible to more securely obtain excellent printing properties and cleanability at 100,000 sheets corresponding time.
For carrying out the image formation method of the present invention, it is preferred that the photosensitive layer consists of a charge generation layer and a charge transfer layer formed thereon, and wherein the hole mobility in the charge transfer layer is a value not less than 1xc3x9710xe2x88x927 cm2/Vxc2x7s in the condition that the electric field strength is about 2xc3x97105 V/cm.
By controlling the hole mobility in the charge transfer layer of the photosensitive layer in this manner, it is possible to more securely obtain excellent printing properties and cleanability even at 100,000 sheets corresponding time.
For carrying out the image formation method of the present invention, it is preferred that the photosensitive layer contains polycarbonate type resin as the binder resin.
Polycarbonate type resin possesses proper hardness and durability so that excellent printing properties and cleanability can be ensured even at 100,000 sheet corresponding time. It is also easy for the photosensitive resin containing polycarbonate type resin as the binder resin to control the hole mobility.
For carrying out the image formation method of the present invention, it is preferred that a developer used in the electrophotography is a two-component developer containing carrier and toner, and wherein the loadings of the toner are a value within a range of 1 to 25 parts by weight when the total amount of the developer is 100 parts by weight.
By making up the developer in this manner, the amount of surplus toner remaining on the OPC photosensitive drum after the transfer to the transfer paper can decrease, achieving excellent printing properties and cleanability. Furthermore, due to its satisfactory cleanability, toner can easily be removed without damaging the OPC photosensitive drum by use of the cleaning blade.
For carrying out the image formation method of the present invention, it is preferred that the surface of the carrier is coated with a high molecular weight material.
By making up the carrier in this manner, a fear that the carrier may damage the OPC photosensitive drum lessens, so that it becomes possible to easily and accurately control the amount X of reduction in the thickness at 10,000 sheets corresponding time.
For carrying out the image formation method of the present invention, it is preferred that the high molecular weight material is a polyolefin type resin.
By using the carrier coated with polyolefin type resin in this manner, a tendency for the carrier to damage the OPC photosensitive drum becomes less.
For carrying out the image formation method of the present invention, it is preferred that the high molecular weight material directly polymerizes monomer onto the surface of the carrier.
By coating the high molecular weight material onto the carrier surface while directly polymerizing the monomers in this manner, it is possible to uniformly coat the high molecular weight material on the carrier surface. Accordingly, a tendency for the carrier to damage the OPC photosensitive drum becomes less.
For carrying out the image formation method of the present invention, it is preferred that the toner in the developer contains coagulated abrasive particles.
By allowing the containment of the coagulated abrasive particles in this manner, it is possible to easily adhere the toner to the OPC photosensitive drum in the development process as well as to easily remove the toner without damaging the OPC photosensitive drum by use of the cleaning blade.
For carrying out the image formation method of the present invention, it is preferred that the toner in the developer contains 0.1 to 10 parts by weight of coagulated abrasive particles when the total amount of the toner is 100 parts by weight.
By setting the content of the coagulated abrasive particles to a value within such a range, it is possible to securely adhere the toner to the OPC photosensitive drum in the development process. Furthermore, by use of the cleaning blade, unnecessary toner can easily be removed without further damaging the OPC photosensitive drum.
For carrying out the image formation method of the present invention, it is preferred that the primary particle diameter of the coagulated abrasive particles is a value within the range of 0.01 to 0.1 xcexcm, and that the secondary particle diameter of the coagulated abrasive particles is a value within the range of 0.2 to 1.0 xcexcm.
By restricting the relationship between the primary particle diameter and the secondary particle diameter in the coagulated abrasive particles, proper hardness and cohesiveness (loosening properties) in the coagulated abrasive particles can be obtained. It is therefore possible to securely adhere the toner to the OPC photosensitive drum in the process of the development, as well as to easily remove unnecessary toner without damaging the OPC photosensitive drum by use of the cleaning blade.
For carrying out the image formation method of the present invention, it is preferred that the coagulated abrasive particles are silica and electrically conductive titania or either of the two.
These coagulated abrasive particles possess proper hardness and cohesiveness. It is therefore possible to more securely adhere the toner to the OPC photosensitive drum in the process of the development. It is also possible to easily remove unnecessary toner without damaging the OPC photosensitive drum by use of the cleaning blade.
For carrying out the image formation method of the present invention, it is preferred that the coagulated abrasive particles adhere to the surface of the toner.
This allows loading of a small amount of coagulated abrasive particles to bring about an excellent abrasive effect.
For carrying out the image formation method of the present invention, it is preferred that the cleaning blade is formed from urethane type rubber having a thickness within the range of 1.5 to 2.0 mm.
Urethane type rubber is superior in durability and resistance to creep, and the urethane type rubber having a thickness within such a range is able to apply an appropriate contact pressure onto the OPC photosensitive drum. It is therefore possible to obtain excellent printing properties and cleanability over a long period of time without causing excessive abrasion of the OPC photosensitive drum attributable to the cleaning blade.
For carrying out the image formation method of the present invention, it is preferred that the position of the cleaning blade is variable so as to correspond to the amount X of reduction in the thickness.
By making up the cleaning blade in this manner, the cleaning blade can be arranged at a position relatively separate from the OPC photosensitive drum when the value of the amount X of reduction in the thickness is small, whereas the cleaning blade can be arranged at a position relatively close to the OPC photosensitive drum when the value of the amount X of reduction in the thickness is large. It is therefore possible to apply at all times a certain appropriate contact pressure to the OPC photosensitive drum, as well as to obtain excellent printing properties and cleanability over a long period of time.
For carrying out the image formation method of the present invention, it is preferred that at least the photosensitive drum, the developing device and the cleaning blade are integrated into a unit.
Use of such a unit facilitates the maintenance, economically ensuring excellent printing properties and cleanability over a long period of time.